Certain types of converters such as modular multi-level converters (M2LC converters), for example, may have a plurality of converter cells which include power-electronic switches by means of which the current to be switched by the converter is switched. In cells such as these, however, one or more of the cells in a converter may need to be bypassed or shorted. This may be the case, for example, when an overcurrent or an overvoltage is present on one cell or a plurality of cells, which can be caused by an external fault (for example, a ground fault) or by an internal fault in the converter cell.
In the event of a significant internal fault in the converter cell, it may be advantageous for the converter cell to be permanently bridged. A pyrotechnic make contact (WO 2009/092621 A1), a vacuum interrupter (WO 2008/125494 A1), or else broken-down semiconductor components (DE 103 232 20 A1, WO 2007/023064) have already been proposed for this purpose.
In the event of external faults, it may be necessary to pass surge currents briefly through the bridging unit and for such surge currents to have to bypass the converter cell. For this purpose, it has been proposed that, for example, a thyristor be arranged in parallel with the freewheeling diode for the converter switches (WO 2008/067786 A1).
Non-resettable bridging units (for example, a pyrotechnic make contact or broken-down components) must be removed from the converter cells, and replaced by new bridging units which have not yet been used, and this involves effort. This removal and replacement process may be highly time-consuming and costly.
Thyristors are also subject to the problem that the bridging unit returns to the open state without external power. A complete power failure of the converter would in this case also switch the bridging units of defective cells back to the switched-off state again.
GB 1 163 507 A specifies a bridging unit of this generic type for shorting a first input and a second input of a converter cell in a converter. Furthermore, U.S. Pat. No. 5,986,909 discloses a bridging unit for a converter cell by means of a relay.